The present invention relates to an improved PVO-zinc activated, lithium modified catalyst containing an oxide of molybdenum for use the in the partial oxidation of hydrocarbons to prepare dicarboxylic acids and anhydrides. Most particularly, the invention relates to the preparation of the improved molybdenum containing catalyst.
Basically, all of the methods used to prepare oxidation catalysts seek to obtain vanadium in a valence state of less than +5. One method of achieving this is to begin with vanadium in less than the +5 valence state. Another method and that used most widely in the art is to start with vanadium in the +5 state and reduce the valency to less than +5. This invention relates to the latter method. Several variations on this method have been used to obtain these catalyst. In one method V.sub.2 O.sub.5 is reduced in a solution with HCl to obtain vanadyl chloride. A typical catalyst preparation may involve dissolving the vanadium, phosphorus, and other components in a common solvent. The reduced vanadium with a valence of less than 5 is obtained by initially using a vanadium compound with a valence of plus 5 such as V.sub.2 O.sub.5 and thereafter reducing to the lower valence with for example, hydrochloric acid during the catalyst preparation to form the vanadium oxysalt, vanadyl chloride, in situ. The vanadium compound is dissolved in a reducing solvent, such as hydrochloric acid, which solvent functions not only to form a solvent for the reaction, but also to reduce the valence of the vanadium compound to a valence of less than 5. Preferably, the vanadium compound is first dissolved in the solvent and thereafter the phosphorus and other components, if any, are added. The reaction to form the complex may be accelerated by the application of heat. The complex formed is then, without a precipitation step, deposited as a solution onto a carrier and dried. Generally, the average valence of the vanadium will be between about plus 2.5 and 4.6 at the time of deposition onto the carrier.
In another method the catalyst is prepared by precipitating the metal compounds, either with or without a carrier, from a colloidal dispersion of the ingredients in an inert liquid. In some instances the catalyst may be deposited as molten metal compounds onto a carrier. The catalysts have also been prepared by heating and mixing anhydrous forms of phosphorus acids with vanadium compounds and other components. In any of the methods of preparation, heat may be applied to accelerate the formation of the complex.
A method of obtaining vanadyl chloride was disclosed by Koppel et al, Zeit. anorg. Chem, 45, p. 346-351, 1905 by the reduction of V.sub.2 O.sub.5 in alcoholic HCl solution. This method has been recommended for the preparation of the phosphorus-vanadium oxidation catalyst for example, by Kerr in U.S. Pat. No. 3,255,211 where the solvent also serves as the reducing agent. Subsequently, U.S. Pat. No. 4,017,521; 4,043,943; 4,251,390; 4,283,307 and 4,418,003 for example, employed this method generally referred to as the "anhydrous process" of reducing vanadium to prepare the basic phosphorus-vanadium catalyst. The catalysts produced by this latter method have been found to be generally superior to similar catalyst by the other methods. Specifically what had occurred to this class of oxidation catalysts prior to the return to the anhydrous process had been the addition of a veritable cornucopia of elements to the base vanadium-phosphorus composition, see for example U.S. Pat. No. 4,105,586 where in addition to V, P and O the catalyst must contain nine other elements. The catalyst were satisfactory, but manufacturing was difficult because of the number of components and their varying effects on the catalyst performance.
Many references disclose oxidation catalysts which are suitable for producing maleic anhydride by the partial oxidation of n-butane, which catalysts contain molybdenum as one component of a phosphorus, vanadium mixed oxide catalyst. For example U.S. Pat. No. 3,980,585 discloses a catalyst containing P, V Cu and one of Te, Zr, Ni, Ce, W, Pd, Ag, Mn, Cr, Zn, Mo, Re, Sn, La, Hf Ta, Th, Ca, U or Sn; and U.S. Pat. No. 4,056,487 discloses a PVO catalyst containing Nb, Cu, Mo, Ni, Co and plus one or more of Ce, Nd, Ba, Hf, U, Ru, Re, Li or Mg. U.S. Pat. No. 4,515,904 discloses a procedure for preparing PVO catalysts which may include one metal of Mo, Zn, W, U, Sn, Bi, Ti, Zr, Ni, Cr or Co in atomic ratios of metal: V of 0.001 to 0.2:1.
U.S. Pat. No. 4,147,661 discloses high surface area PVO mixed oxide catalyst additionally containing W, Sb, Ni and/or Mo at atomic ratios of 0.0025 to 1:1 to vanadium.
U.S. Pat. No. 4,418,003 discloses PVO catalysts containing either Zn or Mo which is deactivated by Na or Li and which may also contain Zr, Ni, Ce, Cr, Mn, Ni ana Al.
Commonly assigned U.S. Pat. No. 5,070,060 discloses an oxidation catalyst which contains molybdenum which is substituted for some of the phosphorous and produces a more stable catalyst.
In all of the disclosed molybdenum containing catalysts the molybdenum and any other metal are added as the oxide or salt of the metal directly to the reaction mixture during formation of the catalyst. While some of the metal compounds, especially the chloride salts, are readily dissolved in the reaction mixture, the molybdenum oxide does not dissolve well and is not fully dispersed in the catalyst.
It is a feature of the present invention that better dispersion of the molybdenum throughout the catalyst is obtained which results in higher conversions and selectivities.